Printed wiring board and method for producing the same

ABSTRACT

A printed wiring board reduced in weight by reducing the size and the thickness of a substrate in its entirety. The printed wiring board includes a rigid substrate  2,  comprised of a core material  11  at least one side of which carries a land  23,  and flexible substrates  3, 4, 5  and  6  comprised of core materials  33, 36  on at least one surface of which a bump  32  for electrical connection to the land  38  is formed protuberantly. The rigid substrate  2  and the flexible substrates  3  to  6  are molded as one with each other, with the interposition of an adhesive in-between, so that the land and the bump face each other.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention relates to a printed wiring board having a rigidsubstrate as a core material, and on at least one surface of which islayered a flexible substrate, and to a manufacturing method for theprinted wiring board.

[0003] 2. Description of Related Art

[0004] Among the printed wiring boards, known so far, there are aso-called rigid substrate, carrying a tough core material, formed e.g.,of a glass epoxy resin, with the core material carrying a wiringpattern, and a so-called flexible substrate, having a flexible corematerial carrying a wiring pattern.

[0005] Meanwhile, in a portable high-performance electronic equipment,such as a digital mobile equipment, in which a demand is raised forincreasing the transmission rate and the memory capacity, there is alsoraised a demand for reducing the size and weight of the equipment inorder to improve its portability. In keeping up with this demand, it isrequired of the printed wiring board to increase the number of I/O pinsand to reduce the weight of semiconductor.

[0006] Among the aforementioned printed wiring boards, there is amulti-layered printed wiring board having plural substrates layeredtogether and plural electrically conductive layers.

[0007] However, in a multi-layered rigid substrate, a through-hole needsto be bored in each substrate for establishing electrical connectionacross the respective conductive layers. If this through-hole is formed,the conductive layer provided on the core material and the inner wallsurface of the through-hole need to be plated. For this reason, in amulti-layered rigid substrate, the copper foils, forming the respectiveconductive layers, cannot be reduced in thickness, so that difficultiesare encountered in further reducing the thickness and weight of theentire substrate or in forming fine patterns thereon. Moreover, in themulti-layered rigid substrate, a wiring pattern is formed on eachsurface of the core material, a further core material is bonded on eachsurface of the core material, now carrying the wiring pattern, and awiring pattern is formed on the so-bonded core materials. Thus, ifmalfunctions occur in the course of the manufacturing process, thesubstrates bonded previously need to be discarded in their entirety,thus decreasing the yield to render it difficult to improve theproduction efficiency or to reduce the production cost.

SUMMARY OF THE INVENTION

[0008] It is therefore an object of the present invention to provide aprinted wiring board which can be reduced in weight by reducing the sizeand the weight of the entire substrate, and a method for producing thisprinted wiring board.

[0009] In one aspect, the present invention provides a printed wiringboard including a rigid substrate having a land on at least one surfaceof a core material, and a flexible substrate including a bump,protuberantly formed on at least one surface of an insulating layer, forestablishing electrical connection to the land, and a land on its othersurface, in which the rigid substrate and the flexible substrate aremolded as one with each other, with the interposition of an adhesivein-between, so that the land and the bump face each other.

[0010] In another aspect, the present invention provides a method forproducing a printed wiring board including the steps of forming a rigidsubstrate including a land on at least one surface of a core material,forming a flexible substrate including a bump, protuberantly formed onat least one surface of anti-insulating layer for establishingelectrical connection to the land, and a land on its other surface, andmolding the rigid substrate and the flexible substrate together byvacuum hot pressuring, with the interposition of an adhesive in-between,so that the land and the bump face each other.

[0011] In the printed wiring board according to the present invention,in which a flexible substrate is layered on at least one surface of arigid substrate exhibiting toughness, it is possible to provide morecircuits than is possible with a layered rigid substrate, thus enablingthe entire device to be reduced in size. The printed wiring board isproduced by bonding a flexible substrate to the rigid substrateexhibiting toughness, so that bonding may be easier than if flexiblesubstrates are bonded together. Moreover, with the present printedwiring board, obtained on bonding the rigid substrate and the flexiblesubstrate, carrying completed respective wiring patterns, electricalconnection can be made to any desired layer in contradistinction fromthe case of a multi-layered rigid substrate where there are providedplural conductive layers.

[0012] In addition, in the method for producing the printed wiringboard, according to the present invention, the technique of formingwiring patterns on both surfaces of the core material, bonding furthercore materials on both sides of the first-stated core material, nowcarrying the wiring patterns, and forming further wiring patterns on thesecond-stated core materials, as in the case of the conventional methodfor producing a multi-layered rigid substrate, is not used, eachsubstrate which is to make up the printed wiring board may be bonded inposition after inspection, thus improving the production yield.Moreover, since the number of times of plating operations for the copperfoil for the flexible substrates is lesser than that for the rigidsubstrate, the flexible substrates may be of lesser thickness than therigid substrate. Since the printed wiring board is produced by layeringthese flexible substrates, it can be of a lesser thickness than thelayered rigid substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013]FIG. 1 is a cross-sectional view showing essential portions of aprinted wiring board embodying the present invention.

[0014]FIG. 2 is an exploded cross-sectional view schematically showingthe state of a printed wiring board embodying the present invention.

[0015]FIG. 3 is a cross-sectional view showing essential portions of arigid substrate forming the printed wiring board.

[0016]FIG. 4 is a cross-sectional view showing essential portions of aflexible substrate forming the printed wiring board.

[0017]FIGS. 5A, 5B and 5C illustrate the manufacturing process for arigid substrata, FIG. 5A being a cross-sectional view showing essentialportions of a core material, on both sides of which are provided copperfoils, FIG. 5B being a cross-sectional view schematically showing thestate in which a through-hole is formed in the core material, and thecopper foil is etched, and FIG. 5C being a cross-sectional viewschematically showing the state in which a first plating layer is formedon the copper wire.

[0018]FIGS. 6A, 6B and 6C illustrate the manufacturing process of arigid substrate, FIG. 6A being a cross-sectional view showing essentialportions of the core material carrying copper foils on both substratesides, FIG. 6B being a schematic cross-sectional view showing the statein which a through-hole is bored in the core material and a copper foilis etched, FIG. 6C being a cross-sectional view schematically showingthe state in which a second plating layer is formed on the first platinglayer for stopping the resin; and FIG. 6D being a cross-sectional viewschematically showing the state in which a third plating layer is formedon the second plating layer.

[0019]FIGS. 7A, 7B and 7C illustrate a manufacturing process for aflexible substrate, FIG. 7A being a cross-sectional view schematicallyshowing the state in which a copper film is formed on a carrier film;FIG. 7B being a cross-sectional view schematic view showing the state inwhich the copper foil has been etched to form a wiring pattern andbumps; and FIG. 7C being a cross-sectional view schematically showingthe state in which a first plating layer is formed on the copper foilsurface,

[0020]FIGS. 8A, 8B and 8C illustrate the manufacturing process of theflexible substrate, FIG. 8A being a cross-sectional view schematicallyshowing the state in which a resist is coated on polyamic acid; and FIG.8C being a cross-sectional view schematically showing the state in whichthe resist is removed after light exposure and development and thedistal end of the bump is protruded from polyamic acid 33 a.

[0021]FIGS. 9A to 9C illustrate the manufacturing process of a flexiblesubstrate, FIG. 9A being-a cross-sectional view schematically showingthe state in which the polyamic acid is turned into imide to form afirst insulating layer composed of polyimide; FIG. 9B being across-sectional view schematically showing the state in which a furthercarrier film is bonded to the surface carrying the bump; and FIG. 9Cbeing a cross-sectional view schematically showing the state in which awiring pattern having a patterned copper foil is formed.

[0022]FIGS. 10A, 10B and 10C illustrate the manufacturing process of aflexible substrate, FIG. 10A being a cross-sectional view schematicallyshowing the state in which polyamic acid is coated on the patternedcopper foil; FIG. 10B being a cross-sectional view schematically showingthe state in which polyamic acid has been patterned; and FIG. 10C beinga cross-sectional view schematically showing the state in which thepolyamic acid is turned into imide to form a first insulating layercomposed of polyimide.

[0023]FIGS. 11A and 11B illustrate the process of bonding a rigidsubstrate and a flexible substrate together, FIG. 11A being across-sectional view schematically showing the state in which theflexible substrate as an inner layer substrata is bonded to the rigidsubstrate, and FIG. 11B being a cross-sectional view schematicallyshowing the state in which the flexible substrate as an outer layer isbonded to a flexible substrate as an inner layer.

[0024]FIG. 12 is a graph showing the relation between the thickness ofan adhesive used for bonding the substrates together and the peelingstrength.

[0025]FIG. 13 is a perspective view showing a modification of theprinted wiring board embodying the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0026] Referring to the drawings, a printed wiring board and amanufacturing method therefor, according to the present invention, willbe explained in detail.

[0027] Referring to FIGS. 1 and 2, a rigid substrate 2 is used as a corematerial, on one surface of which flexible substrates 3, 4 are layered,and on the other surface of which flexible substrates 5, 6 are layered.That is, the flexible substrates 3, 5 prove inner layer substrates,bonded to the rigid substrate 2, whilst the flexible substrates 4, 6prove outer layer substrates, bonded to the flexible substrates 3, 5.

[0028] The rigid substrate 2 includes a core material 11, as shown inFIG. 3. This core material 11 is a tough substrate formed byimpregnating a glass cloth with an epoxy resin. For this core material,a combustion retardant compound, mainly composed of nitrogen orphosphorus, is used in place of a halogen compound, such as brominecompound. On both sides of the core material 11 are formed wiringpatterns 12, 13 comprised of copper foils. In the core material 11,carrying the wiring patterns 12, 13, there are formed through-holes 14,14 for establishing electrical connection of the wiring patterns 12, 13formed on the respective surfaces of the core material 11. On the wiringpatterns 12, 13, formed on the respective surfaces of the core material11, and on the inner wall surfaces of the through-holes 14, 14, thereare formed first plating layers 15, 16, such as by an electrolyticcopper plating method or an electroless copper plating method. In thethrough-holes 14, 14, provided with the first plating layers 15, 16, aninsulating resin 17, for example, is charged to flatten out the surfaceof the rigid substrate 2. That is, in the rigid substrate 2, thethrough-hole 14 is charged to provide for reliable contact of bumps ofthe flexible substrates 3, 5 layered on the respective surfaces, whileassuring positive bonding of the flexible substrates 3, 5.

[0029] On the first plating layers 15, 16, there are formed secondplating layers 18, 19 for forming an electrically conductive layer evenon each surface of the insulating resin 17 charged into thethrough-holes 14, 14. The second plating layers 18, 19 are provided onthe surface of the resin 17 to provide for positive electricalconnection to the bumps of the flexible substrates 3, 5. Although theresin 17 here is insulating, an electrically conductive-material may becharged in order to provide for more reliable electrical connection ofthe paste-like wiring patterns 12, 13. On the second plating layers 18,19, there are formed third conductive layers 21, 22 formed by goldplating still higher in electrically conductivity. In the rigidsubstrate 2, constructed as described above, the portions lying abovethe through-holes 4, 4 operate as lands 23, 23 adapted for beingelectrically connected to the bumps of the flexible substrates 3, 5.

[0030] Since the rigid substrate 2, constructed as described above, is atough substrate, flexible substrates 3, 4, 5 and 6 can be easily bondedto its both surfaces, with the rigid substrate 2 as a core material.

[0031] The flexible substrates 3, 4, 5 and 6, layered on the rigidsubstrate 2, as described above, are constructed in the followingmanner. Since the flexible substrates 3, 4, 5 and 6 are of a similarstructure, the flexible substrate 3, layered on one surface of the rigidsubstrate 2, is explained as an example.

[0032] This flexible substrate 3 has a copper foil 31, proving a wiringpattern 30, as shown in FIG. 4. On one surface of the copper foil 31 isformed a first insulating layer 33 formed of a combustion retardantmaterial, such as polyimide. On the copper foil 31 is formed a firstplating layer 34, by nickel plating, for improving the tightness incontact with respect to the first insulating layer 33 formed ofpolyimide. On the other surface of the copper foil 31 is formed a secondinsulating layer 36 formed e.g., of polyimide. The second insulatinglayer 36 is patterned to form an opening 37 for exposing the copper foil31 to outside to form lands 38, 38 thereat. On the distal ends of bumps32, 32 exposed to outside by the first insulating layer 33, and on thesurfaces of the lands 38, 38, there are formed second plating layers 39,39 by gold plating superior in electrically conductivity. As for theflexible substrate 3, reference is had to the corresponding Japaneseapplication by the present inventors (Japanese patent Application2000-210482).

[0033] The lands 38, 38 are provided on the other surface of the copperfoil 31 and specifically are provided e.g., at locations facing thebumps 32. Referring to FIGS. 1 and 2, the bumps 32, 32 of the flexiblesubstrates 3, 5 are connected to the lands 23, 23 of the rigid substrate2, while the bumps 32, 32 of the flexible substrates 4, 6 are connectedto the lands 38, 38 of the flexible substrates 3, 5 layered on the rigidsubstrate 2. On the lands 38, 38 of the flexible substrates 4, 6,forming the outer layers, electronic parts, such as IC chips, aremounted.

[0034] The adhesion of the rigid substrate 2 to the flexible substrates3, 5, that of the flexible substrates 3, 4 and that of the flexiblesubstrates 5, 6 are by adhesives 7, 7, 7, 7, as shown in FIGS. 1 and 2.These substrates 2 to 6 are unified together by vacuum hot press. Sincethe solder temperature of the solder reflow in mounting electroniccomponents reaches approximately 230° C., an adhesive having superiorthermal resistance is used as the adhesives 7, 7, 7, 7. Specifically, anepoxy acrylic adhesive, not employing halogen compounds in order todisplay thermal resistance and in order not to emit toxic substances onincineration, is used as the adhesive 7. The adhesives 7, 7, 7, 7 areformed to a thickness on the order of 12 to 25 μm. That is, if thethickness of the adhesive 7 is less than 12 μm, sufficient adhesionstrength of the substrate cannot be developed, whereas, if it is thickerthan 25 μm, the bumps 32, 32 are not protruded from the adhesive 7, suchthat electrical connection to the lands 23, 38 cannot be realized.Moreover, when cured, the adhesives 7, 7, 7, 7 are cured and contractedin the direction of thickness to draw the neighboring substrates in adirection of approaching each other to assure reliable connectionbetween the land and the bump.

[0035] Since the printed wiring board 1 is comprised of flexiblesubstrates 3 to 6 layered on both surfaces of the rigid substrate 2exhibiting toughness, fine patterns can be formed on the flexiblesubstrates 3, 4, 5, 6 to form more circuits to reduce the overall size.Also, since the printed wiring board 1 is prepared by bonding theflexible substrates 3 to 6 to the rigid substrate 2 exhibitingtoughness, bonding may be easier than if the flexible substrates arebonded to one another. In addition, since the flexible substrates 3 to 6of the printed wiring board 1 are bonded to the rigid substrate 2 havingthe completed wiring pattern thereon, as will be explained subsequently,electrical connection can be made freely to any layer in distinctionfrom the case of the multi-layered rigid substrate 2 having pluralconductive layers. Moreover, since the flexible substrates 3 to 6 arelayered in the present printed wiring board 1 on the rigid substrate 2,the printed wiring board can be reduced in thickness in its entirety.

[0036] Moreover, since the through-hole 4 of the rigid substrate 2 ofthe printed wiring board 1 is charged with the resin 17, the surfaces ofthe flexible substrates 4, 6 are flattened out to assure facilitatedmounting of the electronic components. An addition, since no halogencompound is used in this printed wiring board 1 as the combustionretardant material for the core material 11 of the rigid substrate 2,and polyimide is used in place of the halogen compounds for the flexiblesubstrates 3 to 6, while no halogen compound is used as the adhesive 7,it is possible to generate prevent toxic substances from being producedon incineration.

[0037] The method for producing the above-described printed wiring board1 is explained with reference to the drawings. In this printed wiringboard 1, the flexible substrates 2 to 6 are produced separately from oneanother, as shown in FIG. 2.

[0038] The manufacturing method for the rigid substrate 2, serving as acore of the printed wiring board 1, is first explained. This corematerial 11 is a tough substrate formed by impregnating a glass clothwith an epoxy resin, as shown in FIG. 5A. For this core material, acombustion retardant compound, mainly composed of nitrogen orphosphorus, is used in place of a halogen compound, such as brominecompound, so that no toxic substance is produced on incineration. Onboth surfaces of the core material 11 are bonded conductive layers 12 a,13 a for forming wiring patterns 12, 13. These conductive layers 12 a,13 a are formed by copper foils each being 12 μm in thickness.

[0039] In the core material 11, carrying the conductive layers 12 a, 13a, through-holes 14, 14 for establishing electrical connection betweenthe conductive layers 12 a, 13 a are formed by a drill unit having adrill diameter of 0.2 mm, as shown in FIG. 5B. The core material 11 nowcarrying the through-holes 14, 14, is then-subjected to high pressurewater rinsing to remove burrs left in the through-holes 14, 14. Then,chemical smear removal is carried out to remove burrs in thethrough-holes 14, 14 not removed by the high pressure water rinsing. Forexample, deburring in the through-holes 14, 14 is carried out by asulfuric acid method, a chromic acid method, or by a permanganatemethod. Meanwhile, this chemical smear removal process can be omitted ifsmear can be sufficiently removed by the aforementioned high pressurerinsing. In the core material 11, having the burrs in the through-holes14, 14 removed sufficiently, the conductive layers 12 a, 12 a aresoft-etched in order to reduce the thickness of the rigid substrate 2and in order to form fine patterns.

[0040] On the conductive layers 12 a, 13 a,now having the through-holes14, 14 formed therein, the first plating layers 15, 16 are formed byelectrolytic copper plating method or by the electroless copper platingmethod, in order to provide for electrical connection of the conductivelayers 12 a, 13 a, as shown in FIG. 15C. The first plating layers 15, 16are formed to a thickness of approximately 10 μm so that the thicknessof the conductive layers 12 a, 13 a and the first plating layers 15, 16combined together will be approximately 22 μm.

[0041] Then, as shown in FIG. 6A, an insulating resin 17 is charged intothe through-holes 14, 14 of the core material 11, now carrying the firstplating layers 15, 16, in order to flatten out the surface of the corematerial 11. Thus, in the rigid substrate 2, the through-holes 14, 14are buried with the resin 17 to flatten out the surface of the corematerial 11 to assure reliable contact with the bumps 32, 32 of theflexible substrates 3, 5 on the through-holes 14, 14. The paste-likeconductive material may be charged into the through-holes 14, 14 toassure more reliable electrical connection of the wiring patterns 12,13.

[0042] Then, as shown in FIG. 6B, the first plating layers 15, 16 aresoft-etched to reduce the thickness of the rigid substrate 2 as well asto form a fine pattern. Specifically, the first plating layers 15, 16are soft-etched so that the thicknesses of the conductive layers 12 a,13 a and the first plating layers 15, 16, combined together, will beapproximately 17 μm.

[0043] Then, as shown in FIG. 6C, the second plating layers 18, 19 areformed on the soft-etched first plating layers 15, 16, by theelectrolytic copper plating method or the electroless copper platingmethod, in order to form an electrically conductive layer on the resin17 charged into the through-holes 14, 14 to assure positive electricalconnection with the bumps 32, 32 of the flexible substrates 3, 5.Specifically, the second plating layers 18, 19 are formed to a thicknessof approximately 10 μm so that the overall thickness of the secondplating layers 18, 19 will be approximately 17 to approximately 27 μm.

[0044] Then, as shown in FIG. 6D, a dry film is stuck to the corematerial 11, now carrying the second plating layers 18, 19, exposed tolight, developed and etched by way of performing the patterning. Thesecond plating layers 18, 19 are plated with gold, exhibiting superiorelectrically conductivity, as shown in FIG. 3, to form third conductivelayers 21, 22. This forms lands 23, 23 of the bumps 32, 32 of theflexible substrates 3, 5. The surfaces of the third conductive layers21, 22 then are subjected to the processing of surface oxidation, usinga solvent, by way of blacking, in order to improve the tightness incontact with the adhesive 7 used for bonding the flexible substrates 3,5. In the printed wiring board 1, the-rigid substrate 2, thus completelypatterned, is used as the core material, the flexible substrates 3, 4are layered on one surface of the rigid substrate 2, whilst the flexiblesubstrates 5, 6 are layered on the other surface of the rigid substrate2.

[0045] The manufacturing method for the flexible substrates 3 to 6,layered on the rigid substrate 2, will now be explained for the flexiblesubstrate 3, by way of an example. As for the manufacturing method forthe flexible substrate 3, reference is had to the corresponding Japaneseapplicant by the present inventors (Japanese patent Application2000-210482).

[0046] First, as shown in FIG. 7A, a copper foil 31 for forming a wiringpattern 30 or bumps 32 is stuck to a carrier film 41. Since the copperfoil 31 by itself is puckered or severed, the carrier film 41 is used asa protective film for the copper foil 31 to prevent this from occurring.For example, the carrier film 41 is formed of a synthetic resinmaterial, such as polyethylene terephthalate, to a thickness ofapproximately 200 μm. On this carrier film 41, a copper foil with athickness of approximately 55 μm is stuck by a UV curable adhesive witha thickness of approximately 40 μm. When bonding the copper foil 31 tothe carrier film 41, the UV light is illuminated in an amount of 50% onthe surface of the copper foil 31 and in an amount of 30% on the surfaceof the carrier film 41. That is, the surface of the carrier film 41 isin the semi-cured state to enable the carrier film 41 to be peeled offin a subsequent process.

[0047] Then, as shown in FIG. 7B, a dry film is stuck to the copper foil31, bonded to the carrier film 41, exposed to light and etched by way ofpatterning. This forms protuberant bumps 32 on the copper foil 31. Sincethe bumps 32 are formed by etching the copper foil 31, it is possible toreduce height variations. The surface of the copper foil 31 carrying thebumps 32, 32 are roughed, using a solvent, for improving the tightnessof contact with the first plating layer 34 of nickel (Ni) formed in thenext step.

[0048] Then, as shown in FIG. 7C, the first plating layer 34 is formedon the entire surface of the copper foil 31, carrying the bumps 32, inorder to improve tightness of contact with the first insulating layer ofpolyimide as the core material of the flexible substrate 3.Specifically, this first plating layer 34 is formed to a thickness ofapproximately 1 μm.

[0049] On the first plating layer 34, polyamic acid 33 a is coated, asshown in FIG. 8A. This polyamic acid 33 a is coated on the first platinglayer 34 to a thickness of approximately 200 μm and dried until it issemi-cured. This gives the thickness of the as-dried polyamic acid 33 aequal to approximately 20 μm.

[0050] On the polyamic acid 33 a, formed of semi-cured polyimide, aresist 42 is coated by a gravure roll to an as-dried thickness ofapproximately 8 μm, as shown in FIG. 8B. The resist 42 then is exposedto light and developed for patterning to a pre-set shape. The resistthen is etched with alkali. This etching is executed until the distalends of the bumps 32, 32 ae exposed to outside for connection to thelands 23, 23 of the rigid substrate 2. That is, the resist 42 is of areduced thickness above the bumps 32, 32, so that the distal ends of thebumps are exposed to outside via the polyamic acid 33 a. The resist 42then is peeled off, as shown in FIG. 8C. The carrier film 41 then ispeeled from the copper foil 31, now carrying the semi-cured polyamicacid 33 a.

[0051] The carrier film 41 is peeled form the copper foil 31 beforeturning the polyamic acid 33 a into imide to prevent the adhesive usedfor bonding the copper foil 31 and the carrier film 41 together frombeing cured by the heating for turning into imide and to prevent meltingof the carrier film 41.

[0052] Then, as shown in FIG. 9A, the polyamic acid 33 a is heated atapproximately 350° C, after peeling the carrier film 41 from the copperfoil 31, to turn it into imide, whereby a first insulating layer 33formed of polyimide is deposited.

[0053] Then, as shown in FIG. 9B, a carrier film 43, similar to thecarrier film 41, is bonded on the surface carrying the first insulatinglayer 33, by a UV curable adhesive.

[0054] Then, as shown in FIG. 9C, a dry film is stuck to the copper foil31, exposed to light and further etched, by way of patterning. Thisforms a wiring pattern 30 on the copper foil 31, while forming lands 38,38 at the sites facing the bumps 38, 38.

[0055] Then, as shown in FIG. 10A, polyamic acid 36 a is coated on thepatterned copper foil 31, as shown in FIG. 10A. This polyamic acid 36 ais dried until it is semi-cured. This gives the thickness of theas-dried polyamic acid 36 a of approximately 8 μm.

[0056] On the polyamic acid 36 a, a resist, not shown, is coated,exposed, developed for patterning to a pre-set shape and subsequentlyetched with alkali. This exposes a portion of the copper foil 31 tooutside to form lands 38, 38, as shown in FIG. 10B. The resist then ispeeled off and the carrier film 43 then is peeled from the copper foil31. Meanwhile, the carrier film 43 is peeled from the copper foil 31before the polyamic acid 36 a is turned into imide to prevent theadhesive used for bonding the copper foil 31 and the carrier film 43from being cured due to heating at the time of conversion to imide, aswell as to prevent the melting of the carrier film 43.

[0057] Then, as shown in FIG. 10C, the polyamic acid 36 a is heated toapproximately 320° C., for conversion to imide, after peeling off thecarrier film 43 from the copper foil 31, whereby the second insulatinglayer 36 formed of polyimide is formed. The lands 38, 38 are then platedwith gold, superior in electrically conductivity, to form a secondplating layer 39, to permit layering on the rigid substrate 2, as shownin FIG. 4. Meanwhile, since polyimide forming the first and secondinsulating layers 33, 36 is of the polycondensation type, it is inferiorin wettability. So, the surface of the first insulating layer 33, formedof polyimide, is processed with potassium permanagnate or O₂ plasma toimprove adhesion characteristics.

[0058] For forming the bumps 32 on the c32, a plating method may also beused in place of the above-described etching method. Specifically, it ispossible to form an insulating layer on the copper foil 31, to form aland-forming area by patterning the insulating layer and to allow thegrowth of copper in the land-forming area by the electrolytic copperplating method to form the bumps 32.

[0059] The bonding step of bonding the flexible substrates 3 to 6 to therigid substrate 2, manufactured as described above, is hereinafterexplained by referring to the drawings.

[0060] In this bonding step, the adhesive 7 is first coated on thesurface of the flexible substrate 4 towards the first insulating layer33, bonded to the other surface of the rigid substrate 2, as shown inFIG. 11A. The flexible substrate 4, thus coated with the adhesive 7, isstuck to the surface of the rigid substrate 2 towards the wiring pattern13, so that the bump 32 will face the land 23. The adhesive 7 then iscoated on the surface towards the first insulating layer 33 of theflexible substrate 3, bonded to one surface of the rigid substrate 2.The flexible substrate 4, coated with the adhesive 7, is stuck to thesurface towards the wiring pattern 12 of the rigid substrate 2, so thatthe bump 32 will face the land 23. The flexible substrate 6, the firstinsulating layer 33 of F which is coated with the adhesive 7, is thenstuck to the flexible substrate 4, as shown in FIG. 11B, so that thebump 32 will face the land 38. The flexible substrate 5, the firstinsulating layer 33 of which is coated with the adhesive 7, is thenstuck to the flexible substrate 3, so that the bump 32 will face theland 38. When the flexible substrates 3 to 6 are stuck in this manner tothe flexible substrate 2, the flexible substrates 3 to 6 are stuck tothe rigid substrate 2 exhibiting toughness, so that the bonding can beachieved readily without puckering.

[0061] Meanwhile, an anisotropic conduction film may also be providedbetween the bumps 32 and the lands 23, 32.

[0062] In view of pressing in the next step, a pressure sensitiveadhesive is used for the adhesive 7. Since the solder temperature of thesolder reflow in mounting electronic components reaches approximately230° C., an adhesive superior in thermal resistance is used.Specifically, an epoxy acrylic adhesive is used as this adhesive 7. Theadhesives 7, 7, 7, 7 are each formed to a thickness of 12 to 25 μm. Thatis, when the substrates are stuck to one another, as shown in FIG. 12,the peeling strength not less than approximately 1000 g/cm is required.So, the adhesive 7 needs to be of such a thickness as assures theminimum peeling strength, that is 12 μm or more. It is noted that thepeeling strength-of the adhesive 7 tends to be the higher the thickerbecomes the adhesive layer. On the other hand, the thickness of theadhesive 7 needs to be sufficient to permit the bumps 32 to be protrudedfrom the adhesive 7 when the substrates are bonded together. Thus, thethickness of the adhesive 7 is set to not more than 25 μm to assurereliable electrical connection of the lands 23, 38 with the bumps 32.

[0063] When the flexible substrates 3 to 6 are stuck to the rigidsubstrate 2 by the adhesive 7, these substrates are pressed together bya vacuum hot pressuring method. For example, this vacuum hot pressuringis performed under a condition of 180° C., 120 minutes and 40 kg/cm²,the vacuum hot pressuring method is used to prevent air voids from beingformed between the neighboring substrates during pressuring. Theultrasonic welding method, while being superior in the ohmic contact, isinferior to the vacuum hot pressuring method in making large-areaconnection. Meanwhile, the ultrasonic welding method may be applied toassure reliable ohmic contact after the vacuum hot pressuring.

[0064] The substrate prepared by bonding the flexible substrates 3 to 6to the rigid substrate 2 by the adhesive 7 and by vacuum hot pressuring,is exteriorly worked by a metal mold pres or the router to complete theprinted wiring board 1 shown in FIG. 1. On the printed wiring board 1,thus prepared, electronic components are mounted by a method such assolder reflow method. Since the through-hole 14 of the rigid substrate 2is filled with the resin 17, the outer layers of the flexible substrates5, 6 are smoothed to permit reliable mounting of the electroniccomponents.

[0065] Table 1 below shows an illustrative manufacture of the printedwiring board 1. TABLE 1 inventive conventional flexible wiring patternpattern width 25  50 substrate (μm) pattern 25  50 interval (μm) innerlayer land diameter 70 500 connection (μm) bump 35 200 diameter (μm)insulating material polyimide polyimide layer thickness 30  60 rigidwiring pattern pattern width 50 100 substrate (μm) pattern 50 100interval (μm) inner layer land diameter 200 300 connection (μm) bump 150150 diameter (μm) insulating material glass glass epoxy layer epoxythickness 100 130

[0066] As may be seen from Table 1, the wiring patterns 12, 13 and thelands 23 of the rigid substrate 2 and the wiring pattern 30 and thelands 38 of the flexible substrates 3 to 6 are finer than in theconventional rigid substrate or flexible substrate. It is thus seenthat, in the printed wiring board 1 according to the present invention,the wiring pattern may be formed to a higher density in the printedwiring board 1 of the present invention than if plural rigid substratesare stacked to provide a multi-layered printed wiring board havingplural conductive layers, as conventionally.

[0067] In the above-described manufacturing method for the printedwiring board 1, the rigid substrate 2 and the printed substrates 3 to 6are prepared separately and unified ultimately together by vacuum hotpressuring to produce the printed wiring board 1 reduced in size andthickness as shown in FIG. 1. Since the technique of forming wiringpatterns on both surfaces of the core material, bonding further corematerials on both sides of the first-stated core material, now carryingthe wiring patterns, and forming further wiring patterns on thesecond-stated core materials, such as is used in the conventional methodfor producing a multi-layered rigid substrate, is not used, eachsubstrate which is to make up the printed wiring board 1 can be bondedin position after inspection, thus improving the production yield.Moreover, since the number of times of plating operations for the copperfoil 31 in the case of the flexible substrates 3 to 6 is lesser thanthat in the case of the rigid substrate, the flexible substrates 3 to 6may be of lesser thickness than the rigid substrate 2. Since the printedwiring board 1 is produced by layering these flexible substrates, it canbe of a lesser thickness than the layered rigid substrate.

[0068] Referring to FIG. 13, a modified embodiment of the printed wiringboard according to the present invention is hereinafter explained. Aprinted wiring board 50, shown in FIG. 13, features layering flexiblesubstrates 52 to 55 on both surfaces of a rigid substrate 51, a midportion 56 of which is exposed to outside as shown. On one side of therigid substrate 51, there is layered the flexible substrate 52, whereas,on the opposite side of the rigid substrate 51, there are layered theflexible substrates 53 to 55. The flexible substrates 52 to 55 arebonded by the above-mentioned adhesive 7 to the rigid substrate 51 andare unified thereto on vacuum hot pressuring. In the printed wiringboard 51, electronic components can be mounted on the exposed portions56 on both sides of the rigid substrate 51, whilst further electroniccomponents may be mounted on the flexible substrates 52, 55 forming theouter layers of the printed wiring board 50. One of the flexiblesubstrates 52 to 55 may also be extended outwards from the substrateproper so as to be used for connection to the other electroniccomponents. The method for producing the rigid substrate 51 and theflexible substrates 52 to 55 is similar to that for the rigid substrate2 and the flexible substrate 3, described above, and hence is notexplained specifically.

[0069] The present invention is not limited to the printed wiring board1 explained above with reference to the drawings. For example, the rigidsubstrate 2 is not limited to a double side printed wiring boardcarrying wiring patterns 12, 13 on its both sides, such that amulti-layered rigid substrate carrying three or more conductive layersmay also be used. The flexible substrate layered on the rigid substrate2 may be formed in one or more layers on one or both sides of the rigidsubstrate 2.

What is claimed is:
 1. A printed wiring board comprising: a rigidsubstrate including a land on at least one surface of a core material;and a flexible substrate including a bump, protuberantly formed on atleast one surface of an insulating layer for establishing electricalconnection to said land, and a land on the other surface thereof; saidrigid substrate and the flexible substrate being molded as one with eachother, with the interposition of an adhesive in-between, so that saidland and the bump face each other.
 2. The printed wiring board accordingto claim 1 wherein said insulating layer of said flexible substrate ispolyimide.
 3. The printed wiring board according to claim 1 wherein bothsides of said rigid substrate carry said lands and wherein there arelayered said flexible substrates on both sides of the rigid substrate.4. The printed wiring board according to claim 1 wherein said flexiblesubstrates are layered on both sides of said rigid substrate and whereina portion of said rigid substrate is exposed to outside to form anexposed portion.
 5. The printed wiring board according to claim 1wherein said flexible substrate includes a copper foil operating as awiring pattern, said bump is provided on one surface of said copperfoil, a first plating layer is formed on the entire surface of saidcopper foil carrying said bump, a first insulating layer is formed onsaid first plating layer excluding an area thereof corresponding to saidbump, said land is formed on the other surface of said copper foil andwherein a second insulating layer is formed on said other surface, saidsecond insulating surface having an opening facing said land having asecond plating layer.
 6. A method for producing a printed wiring boardcomprising the steps of: forming a rigid substrate including a land onat least one surface of a core material; forming a flexible substrateincluding a bump, protuberantly formed on at least one surface of aninsulating layer for establishing electrical connection to said land,and a land on the other surface thereof; and molding said rigidsubstrate and the flexible substrate together by vacuum hot pressuring,with the interposition of an adhesive in-between, so that said land andthe bump face each other.
 7. The method for producing a printed wiringboard according to claim 6 further comprising the steps of: forming athrough-hole in said core material provided with conductive layers onboth sides thereof as a wiring pattern has been formed on each surfaceof said rigid substrate; plating the-entire surface of said corematerial inclusive of said through-hole; burying said through-holehaving a plating layer formed on the inner wall surface thereof; furtherplating said core material 11 having the through-hole buried; andpatterning said plated conductive layer to form a wiring pattern and theland electrically connected to said bump.
 8. The method for producing aprinted wiring board according to claim 6 wherein said step of formingsaid flexible substrate includes the steps of: providing said bump onone surface of a copper foil serving as a wiring pattern by patterning;forming a first plating layer on the entire surface of said copper foilcarrying said bump; forming a first insulating layer on said firstplating layer; patterning said first insulating layer to expose saidbump; patterning the other surface of said copper foil to form saidland; forming a second insulating layer on said land; patterning saidsecond insulating layer to form an opening facing said land; and forminga second plating layer on said land of said opening.
 9. The method forproducing a printed wiring board according to claim 8 wherein said stepof forming said first and second insulating layers includes the step ofcoating and forming polyamic acid, forming a pre-set pattern in a coatedlayer of said polyamic acid and heating and curing said polyamic acidfor conversion to an imide.
 10. The method for producing a printedwiring board according to claim 6 wherein said rigid substrate and theflexible substrate are unified together by vacuum heat pressuringfollowed by ultrasonic welding.